WP 4.9 Pollyfill JS Compatablity.

[milindmore22]

Bug Description

AMP Settings screen is inaccessible with WP 4.9.x versions. due to polyfill js dependencies as old version required to attach it to global windows object.

This also brings the question of if we should drop support for older WP versions 4.9? it's been more than 2 years since Gutenberg is released.

The console error

Uncaught TypeError: Cannot read property 'i18n' of undefined
    at wp-api-fetch.js?ver=7645fa7ef83c5d94141bdac792ab796a:1
    at Object.2 (wp-api-fetch.js?ver=7645fa7ef83c5d94141bdac792ab796a:1)
    at r (wp-api-fetch.js?ver=7645fa7ef83c5d94141bdac792ab796a:1)
    at Object.48 (wp-api-fetch.js?ver=7645fa7ef83c5d94141bdac792ab796a:1)
    at r (wp-api-fetch.js?ver=7645fa7ef83c5d94141bdac792ab796a:1)
    at Module.<anonymous> (wp-api-fetch.js?ver=7645fa7ef83c5d94141bdac792ab796a:1)
    at Module.216 (wp-api-fetch.js?ver=7645fa7ef83c5d94141bdac792ab796a:1)
    at r (wp-api-fetch.js?ver=7645fa7ef83c5d94141bdac792ab796a:1)
    at Object.215 (wp-api-fetch.js?ver=7645fa7ef83c5d94141bdac792ab796a:1)
    at r (wp-api-fetch.js?ver=7645fa7ef83c5d94141bdac792ab796a:1)
admin.php?page=amp-options:81 

Uncaught ReferenceError: wp is not defined at admin.php?page=amp-options:81

Expected Behaviour

AMP Settings screen Works as expected with the WP 4.9

Steps to reproduce

1. Setup WP 4.9 or you can use WP Downgrade Plugin and set up 4.9
2. Install AMP if it's already not present.
3. Visit the AMP settings screen.
4. Open Browser Console and See error

Screenshots

Additional context

• WordPress version: 4.9.x
• Plugin version: 2.0.11
• Gutenberg plugin version (if applicable): Nope we are going Classic
• AMP plugin template mode: Transional Mode
• PHP version: 7.4
• OS: Mac OS
• Browser: chrome
• Device: Macbook Air

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[delawski]

I've done some research today and found out that the Settings page stopped working on WordPress 4.9 after releasing AMP 2.0.5.

I'm almost certain it has something to do with #5461. Most likely I won't have time to look into this today, but it's on my to-do list for tomorrow.

[westonruter]

@delawski This change may be the cause: 05d751a (via #5461).

[delawski]

@westonruter As @pierlon found out and fixed in #6119, the issue was caused by the fact that the wp-i18n package now depends on wp-hooks (see WordPress/gutenberg#27966). The PR is now ready for your review/QA and merge.

[westonruter]

QA Passed

On 4.9, accessing the setting screen results in:

Upon upgrading to 2.1 on develop, no such error: